Setting a when Breakpoint
at a Line

A when breakpoint command accepts other dbx commands such as list, letting you write your
own version of trace.

(dbx) when at 123 {list $lineno;}

The when command operates with an implied cont command.
In the example above, after listing the source code at the current line, the
program continues executing. If you included a stop command
after the list command, the program would not continue
executing.